Side post reinforcing plate of railway wagon is a typical medium and growth stamping plate made of  Q450NQR1. Based on the theoretical analysis, stamping process was optimized and designed which could increase the service performance and manufacturability of the side post reinforcing plate. The structure and operation process of oblique cutting die were described in details. The practical processing shows that the die is simple , easy, safe, reliable to be operated and the produced workpiece is qualified. Tens of thousands of tarpaulin guard irons have been produced by the die and the economic benefits are excellent.